The delayed coking process is an established petroleum refinery process which is used on very heavy low value residuum feeds to obtain lower boiling cracked products. It can be considered a high severity thermal cracking or destructive distillation and may be used on residuum feedstocks containing nonvolatile asphaltic materials which are not suitable for catalytic cracking operations because of their propensity for catalyst fouling or for catalyst deactivation by their content of ash or metals. Coking is generally used on heavy oils, especially vacuum residua, to make lighter components that can then be processed catalytically to form products of higher economic value. In the delayed coking process, the heavy oil feedstock is heated rapidly in a fired heater or tubular furnace from which it flows directly to a large coking drum which is maintained under conditions at which coking occurs, generally with temperatures above about 450.degree. C. under a slight superatmospheric pressure. In the drum, the heated feed decomposes to form coke and volatile components which are removed from the top of the drum and passed to a fractionator. When the coke drum is full of solid coke, the feed is switched to another drum and the full drum is cooled and emptied of the coke product. Generally, at least two coking drums are used so that one drum is being charged while coke is being removed from the other.
When the coking drum is full of solid coke, the hydrocarbon vapors are purged from the drum with steam at temperatures in excess of about 800.degree. F. The drum is then quenched with quench water to lower the temperature to below about 200.degree. F. after which the water is drained. When the cooling step is complete, the drum is opened and the coke is removed by mining or cutting.
Hydraulic decoking is currently used to remove solidified coke from the coking drum. This process uses a high speed, high impact water jet which cuts the coke from the coking drum. Boring and cutting tools, each producing several jets of water from high pressure nozzles, are employed to mine the coke. A hole is bored in the coke and the cutting head, which follows, breaks up the coke.
The boring tool has jet nozzles which are oriented vertically downward. This tool hydraulically makes a pilot hole of about 2-3 feet in diameter from the top of the coke downwards. The cutting tool has jet nozzles oriented horizontally and cuts the coke from the drum, traveling from the top downwards or from the bottom upwards.
The cooling step is very important from the standpoint of safe operation of the unit. Decoking the unit before it is adequately cooled can subject operating personnel to hazardous steam and fumes.
Even though the coking drum may appear to be completely cooled, occasionally, a problem arises which is referred to in the art as a "hot drum". This problem occurs when a localized zone in the drum does not completely cool. It is believed that the quench water cannot effectively penetrate the zone because of the properties of the coke which makes the zone impervious to the quench water. Because the quench water cannot reach the impervious zone, it is not cooled as quickly as the rest of the coking drum resulting in a localized region of high temperatures. This condition is difficult to detect and may not be noticed by operating personnel.
The solution to the problem of a hot drum has been approached in the past by cofeeding a heavy, aromatic hydrocarbon feed which keeps the viscosity of the coker feed low during coking to reduce the occurrence of a high viscosity section which is believed to create an impervious zone. However, this technique has a limited advantage since cokers are, typically, refinery bottlenecks and sending these cofeeds, which include heavy aromatic oils, clarified slurry oils or FCC bottoms fractions, to the coker reduces the overall amount of crude which can be used in the refinery to make gasoline and other more valuable products.
Another problem encountered in the delayed coking process is that prior to decoking the drum, the drum is kept out of operation for extensive periods of time which are necessary to assure adequate cooling. A process in which the time required to cool the coking drum is reduced would be an improvement in the delayed coking process overall because having the drum out of operation lowers the coking capacity of the plant and increase the amount of quench water consumed during the cooling step.